An electric generator transforms rotational energy into electrical energy according to generator-action principles of a dynamoelectric machine. Turning torque is supplied to a rotating and magnetized rotor by a combustion or steam-driven turbine and converted to alternating current (AC) electricity, typically three-phase AC, in a stationary shell-like cylindrical stator. Rotation of the rotor within an axial bore of the stator generates AC electricity within stator windings supported by a stator core.
The generator is a mechanically massive and electrically complex structure, supplying output power up to 2,222 MVA at voltages up to 27 kilovolts. A large generator, for example a 500 megawatt generator, weighs about 200 tons, is approximately 6 meters long and 2.6 meters in diameter, with a bore diameter of about 1.3 meters and an air gap (i.e., between the rotor and stator) of about 0.75 to about 2.0 inches. Electrical generators are the primary power producers in an electrical power system.
The stator core comprises thousands of thin high-permeability (e.g., steel) circumferentially-slotted laminations (about 200,000 laminations in one embodiment) that are horizontally stacked and clamped together. Each lamination defines a central opening and thus when stacked the plurality of openings define the axial bore that extends an axial length of the core. The plurality of laminations defines the stator core. Each lamination is about 0.3 mm thick and coated with an insulating material, for example a varnish, to electrically insulate each lamination from adjacent laminations that it contacts and thereby reduce eddy current losses. The core laminations are held together by bars or rods that are distributed around a circumference of the core and extend axially through each lamination.
Each lamination (and thus the stator core) further comprises a plurality of inwardly facing (i.e., toward a centerline of the core) teeth. Stator windings, typically comprising electrically insulated copper bars, are disposed within parallel slots that are defined between consecutive teeth. The copper bars extend axially along a length of the core. The generator output current is generated within these copper bars.
The rotor is rotatably driven by a rotating turbine and carries an axial field winding (also referred to as a rotor winding) energized by direct current supplied from an exciter. As the constant (with respect to time) magnetic flux produced by the rotor winding rotates within the stator core, it cuts the stator windings and generates alternating current within these windings. The steel laminations ensure that the stator core presents a path of low magnetic impedance to the magnetic flux of the spinning rotor.
The rotor and stator are enclosed within a frame. Each rotor end comprises a bearing journal cooperating with bearings attached to the frame to provide a low-friction interface between the rotor and the frame.
The AC electricity induced in the stator windings by action of the rotor's rotating magnetic field flows to external terminals on the generator frame for connection to an external electrical load. Three-phase alternating current is produced by a generator that comprises three independent stator windings spaced at 120° around the stator core. Single-phase alternating current is supplied from a single stator winding.
It is vital to prevent the generation of unwanted currents in the stator core (as opposed to the desired currents in the stator windings) that may cause serious core overheating, explosion, or fire if not detected and repaired. The insulation between adjacent laminations is intended to prevent the formation and flow of these currents. However, if insulation between the laminations, especially insulation along a tooth edge proximate the bore opening, is damaged during assembly, operation or maintenance, conducting circuits may be formed. The rotating flux can induce currents within these circuits; the flow of these currents can cause hot spots (regions of high current density that lead to overheating) in the damaged area. If allowed to persist, the high temperature generated in the region surrounding the hot spot can also damage or possibly lead to failure of electrical insulation surrounding the stator conductors, necessitating replacement of these stator conductors. There have been situations where hot spots have grown so large that the entire core had to be rebuilt.
One prior art hot spot detector, referred to as a loop test, excites the core to a magnetic flux density near its operating flux density (e.g., about 85% of the operating flux density) using a temporary high-power ring flux loop. This technique employs a heavy gauge conductor that extends through the stator bore, around the outside of the generator frame, then back through the bore. Three to ten turns of this conductor are normally required. The loop is energized with a high voltage and technicians are positioned within the bore to manually examine the surface of the stator in search of hot spots.
A thermographic inspection technique is an alternative to conducting hands-on observations. This technique also employs the heavy gauge conductor to excite the core to its operational (or near operational) magnetic flux density. The entire surface of the core is then scanned with an infrared detector. The scan process is conducted from one end of the core to the other end, with the detector traversing axially and circumferentially in search of infrared radiation that reveals core hot spots.
The loop test is typically performed on a new or rewound stator core because the rotor must be removed before the test can be performed. The test provides a baseline result for comparing against subsequent loop tests (or other hot spot detecting tests) conducted on that core. These latter tests may reveal potential hot spots. By comparison with the baseline test results, one can determine whether a particular hot spot has recently developed or was present during the baseline test.
More recently, electromagnetic detectors, such as an Electromagnetic Core Imperfection Detector (EL-CID) as described in U.S. Pat. No. 5,321,362, have been employed to identify core hot spots. This technique employs, in one embodiment, an excitation current loop (usually six turns) of No. 10 AWG 300-volt wire installed in the bore of the stator core. The wire is commonly suspended along a bore center line and around the frame in a manner similar to the path of the conductor used in the high-power loop test technique described above.
The conductor loop is connected to a source of constant-frequency amplitude-adjustable AC voltage (a 240-volt variable transformer, for example). A separate single-turn search coil determines when the proper level of core excitation has been achieved. Typically, the voltage is adjusted to produce a flux density of approximately 4% of the operating flux density of the generator core. At this low flux density, technicians can safely enter the bore with a detector pickup coil (i.e., a Chattock coil or sensor) to detect axial currents in the laminations by detecting magnetic fields emanating from those currents as they flow through the shorted laminations. Alternatively, the pickup coil is remotely controlled to move within the bore, in particular in an application when the rotor is in place when the EL-CID test is conducted.
For conducting tests with the rotor in place, the assignee of the present invention has developed a process (referred to commercially as a FAST GENSM test) in which a robotic carriage carrying the EL-CID sensor is fed into the air gap space between the rotor and the stator. For FAST GEN inspections, the excitation current loop comprises about six or seven turns of flat conductor cable that is also threaded through the air gap.
Whether the EL-CID test is conducted with the rotor in place or removed, the EL-CID pickup coil or sensor is moved over the entire inwardly-facing surface that defines the bore. The sensor is moved in a series of overlapping circumferential patterns to test all coil slots and teeth around the entire 360 degrees circumference and over the entire axial length of the core. The output signal is observed on an output device or plotted. Any areas of elevated axial current in the laminations, whether along the surface of the core that defines the bore or at some distance below that surface, are indicated as peaks in the output signal. The need for corrective action can be determined by analyzing these peaks.
A desired value of the EL-CID excitation voltage is a function of several core and stator parameters, including the stator line-to-line voltage, the number of turns per phase winding, the coil pitch, the number of rotor poles, and the number of stator winding slots. The resulting excitation voltage produces a desired level of magnetic flux that in turn generates a desired voltage in the pick-up coil. This value of flux produces a uniform scalar magnetic potential drop between adjacent teeth of the stator core along the axial length of the core. Hot spots in the core disturb this uniform potential both axially and circumferentially, producing a different potential value that can be detected by the sensor coil.
The output signals from the detector pickup coil can be further processed and analyzed by comparing the output signals to known reference values (e.g., based on earlier scans of the same core, such as a baseline scan) to assist in characterizing any hot spot or flaw that has been identified.
Strong real-time magnetic fields are created during generator operation and during the loop test (which is performed at about 85% of the generator's rated flux); residual magnetic fields are those that remain after the generator has been shutdown or the loop test concluded. The nature and strength of these residual fields are functions of the magnetic properties of the core material, heat treatment of the core material, residual stresses and the manner in which the core was shut down.
In identifying core hot spots (either by conducting a loop test or an EL-CID test) it is desirable to have a hot spot test conducted when the residual magnetism is zero or near zero (e.g., on a new, restacked or rewound core). The results of such a test are referred to a baseline results or flat line results (e.g., minimal or zero residual magnetism, with no noise in the test output caused by residual magnetism). When testing newly-manufactured cores, a “flat line” trace plot is created for each tested coil slot. This plot provides an ideal baseline since the residual magnetism is zero. The baseline can be used later for comparison with all future test results and trend analyses for the generator.
The amount of residual magnetism that remains in the core after core shutdown is neither accurately determinable nor accurately controllable. It is determined from the BH curve (magnetic flux (B) and magnetic field intensity (H) curve) for a specific core and the level of the magnetic field intensity when the generator is shutdown. The amount of residual magnetism remaining after a loop test is also determined from the BH curve for the core.
To understand the cause and effects of the residual magnetism, one can consider the EL-CID excitation loop as a primary transformer coil and the EL-CID sensor as a secondary transformer coil (step down). The stator serves as a transformer core and thus is a primary determinant of transformer efficiency. A demagnetized core (which has a high permeability and a low reluctance since reluctance and permeability are inversely related) is more efficient and homogenized; power is transmitted cleanly (i.e., with little noise) and easily from the primary to the secondary circuit as the magnetic fields pass easily through the stator core. Residual magnetization in the core decreases the core permeability (and therefore increases the reluctance or resistance to the magnetic fields), raises electrical losses in the core and causes fluctuations in the transmission of power between the primary and secondary coils. As a result of this residual magnetism, a small change in the primary coil voltage leads to a large change in the secondary coil voltage. These large voltages mimic signals produced by stator “short circuits,” which the EL-CID test is designed to detect. Thus the residual magnetism in the stator (i.e., the transformer core) masks or exacerbates the EL-CID signals by interfering with the transmission of power between the EL-CID exciter and detector.
Unfortunately, the resulting noisy EL-CID test results (whether in numerical or graphical form) require tedious interpretation and trend analysis to remove the effects due to residual magnetism from the true test results. Results of an EL-CID test performed after a loop test typically show a high noise level due to the residual magnetism that remains after the loop test. Results of the El-CID test performed after generator shutdown also show high noise levels again due to the residual magnetism resulting from generator operation. EL-CID tests performed before the loop test typically indicate a much lower noise signal level, but still a level that is problematic.
The amount of residual magnetism is also dependent on the manner in which the generator is shutdown, i.e., a normal shutdown or a forced shutdown. A normal shutdown typically produces minimal residual magnetism. An emergency shutdown or rapid loss of load (i.e., a forced shutdown) may cause significant residual magnetic fields to be present in the stator.
Since new cores have not been in active service nor subjected to a prior loop test, an EL-CID test can be performed on a new core under near-ideal test conditions, i.e., without the effects of residual magnetism. Comparing the EL-CID test results of a new core with results from the core after it has been in service is difficult due to a possible difference in the amount of residual magnetism at the time of each test. The residual magnetism, when present, fouls the test results, making it difficult to accurately compare the results, conduct trend analyses, and identify further deterioration of hot spots by comparison with prior test results.
Not only have prior EL-CID test results displayed a poor signal-to-noise ratio, they have also exhibited a phenomenon referred to as “banding.” Banding refers to the movement or oscillation of the EL-CID trace plot above and/or below a zero level during a portion of the overall trace. It appears that this “banding” is due to differences in magnetic permeability along the length of the stator core. The “banding” is exacerbated by the presence of residual magnetic fields in the core.
Demagnetization or degaussing of rotating machinery for preventing electrical discharge damage is known in the art. Demagnetization of machinery components after magnetic particle NDE (nondestructive evaluation) is also common industry practice. However, demagnetization of a generator core prior to conducting hot spot testing has heretofore not been successfully accomplished.